(1) Field of the Invention
This invention generally relates to antennas and more specifically to antennas characterized by omnidirectional radiation patterns.
(2) Description of the Prior Art
Numerous communication networks utilize omnidirectional antenna systems to establish communications between various stations in the network. In some networks one or more stations may be mobile while others may be fixed land based or satellite stations. Omnidirectional antenna systems are preferred in such applications because alternative highly directional antenna systems become difficult to apply, particularly at a mobile station that may communicate with both fixed land based and satellite stations. In such applications it is desirable to provide an omnidirectional antenna system that is characterized further by a wide band width, a good front-to-back ratio, right- or left-handed circular polarization and a compact size.
Some prior art omnidirectional antenna systems use an end fed quadifilar helix antenna for satellite communication and a co-mounted dipole antenna for land based communications. However, each antenna has a limited band width and collectively their performance can be dependent upon antenna position relative to a ground plane. The dipole antenna tends to have a low front-to-back ratio that can cause heavy reflections when the antenna is mounted on a ship, particularly over low elevation angles. These co-mounted antennas also have spatial requirements that can limit their use in confined areas aboard ships or similar mobile stations.
The following patents disclose helical antennas that exhibit some, but not all, the previously described desirable characteristics:
U.S. Pat. No. 3,623,113 (1971) Faigen et al.
U.S. Pat. No. 4,644,366 (1987) Scholz
U.S. Pat. No. 5,134,422 (1992) Auriol
U.S. Pat. No. 3,623,113 to Faigen et al. discloses a balanced, tunable, helical mono-pole antenna that operates independently of a ground plane. This antenna utilizes a centrally fed, multiple-turn, helical antenna with a single element. End winding shorting means in the form of "top hat"- or "can"-type housings tune the antenna by changing the active electrical length of the antenna. A feed loop is centrally disposed to the helical mono-pole antenna winding to provide a balanced input to the antenna. Although this antenna is compact and can be tuned through a wide band width, it does not provide an omnidirectional radiation pattern.
U.S. Pat. No. 4,644,366 to Scholz discloses a miniature radio transceiver antenna formed as an inductor wrapped about a printed circuit card. A peripheral conductor on one side of the card provides distributed capacitance to the end of the antenna that cancels inductive effects and broadens band width. A peripheral conductor on the opposite side of the card provides a capacitance to ground to tune the antenna to frequency. An unbalanced transmission line connects between one end of the antenna and a tap or feed point to provide impedance matching and tuning. This antenna has a limited band width for a given connection point. Moreover it does not produce an omnidirectional radiation pattern.
U.S. Pat. No. 5,134,422 to Auriol discloses an antenna with helically wound, equally spaced, radiating elements disposed on a cylindrical surface. Antennas identified as prior art antennas in this reference include helically wound, end driven antenna elements. The other ends of the elements terminate as open circuits. These antennas provide circular polarization, an omnidirectional radiation pattern and a good front-to-back ratio. The Auriol patent is particularly directed to a structure that uses a conductive, meandering strip to connect the driven ends and establish various phase relationships and tuning. This antenna is designed to produce high quality circular polarization, an omnidirectional radiation pattern and a good front-to-back ratio, but only over a narrow frequency band.
The following patents disclose center-fed spiral antennas that exhibit some, but not all, of the previously described desirable characteristics:
U.S. Pat. No. 4,243,993 (1981) Lamberty et al
U.S. Pat. No. 5,053,786 (1991) Silverman et al
U.S. Pat. No. 4,243,993 to Lamberty et al discloses broad band antennas comprising center feed, spiral antenna arms arranged on planar and conical surfaces. Each antenna arm includes one or more choke elements that resonate at a predetermined operating frequency to eliminate or minimize undesired radiation and reception characteristics and provide sum and difference mode operations with both right-hand and left-hand circularly polarized radiation characteristics. Feeding an antenna as disclosed in the Lamberty et al patent with a phased sequence of signals produces a radiation pattern that exhibits a null along an antenna bore sight axis and a maximum field along a cone of revolution about the bore sight axis. Although this antenna has a broad band width and provides circular polarization, it does not provide an omnidirectional radiation pattern.
U.S. Pat. No. 5,053,786 to Silverman et al. discloses a broad band directional antenna in which two contiguous conductive planar spirals are fed at their center. The antenna is positioned near a cavity to absorb rear lobes in order to improve the front-to-back ratio. Even with this improvement in the front-to-back ratio, the antenna provides a relatively narrow beam pattern having both horizontal and vertical polarization. Apparently, this antenna is designed to operate with a linearly polarized, high gain, narrow beam. Thus the antenna does not provide an omnidirectional radiation pattern or circular polarization. Moreover, by absorbing the rear lobes, the power transmitted into the reserve lobes is lost making the antenna less efficient in radiating during a transmitting mode.